Rock Fracture Sorptivity as Related to Aperture Width and Surface Roughness

Abstract

© Soil Science Society of America. Fractures in low-porosity rocks can provide conduits for fluid flow. Numerous researchers have investigated fluid flow through fractures under saturated conditions. However, relatively little information exists on spontaneous imbibition in fractures, whereby a wetting fluid displaces a non-wetting fluid by capillarity. We investigated spontaneous imbibition of water displacing air in a suite of fractured low-porosity sedimentary and igneous rock cores (5.08-cm length by 2.54-cm diameter). Mode I fractures were induced in the cores by compression between opposing parallel flat plates. The following physical properties were measured: bulk density, r b ; solid-phase density, r s ; porosity, f; contact angle, q e ; fracture aperture width, x geo; and fracture surface roughness, W r . The wetting front in each fracture was imaged using dynamic neutron radiography. Early-time uptake exhibited a square root of time dependency, and was quantified by linear regression, with the slope equal to the fracture sorptivity, S f . Estimates of S f ranged from 10.1 to 40.5 mm s −0.5 , with a median value of 25.0 mm s −0.5 . There was a statistically significant effect of rock type on S f , with igneous rocks generally having lower mean values than sedimentary rocks. Differences in r b , r s , f, and q e between the rock types did not contribute significantly to the variation in S f . However, x geo and W r were significantly correlated with S f . These correlations indicated that S f increases with increasing x geo , as predicted by early-time capillary theory, and decreases with increasing W r , analogous to the decrease in fracture permeability with increasing surface roughness observed under saturated flow conditions.